What are the effects of sulphur oxides on human health and ecosystems?

95% of the SOx emitted from the combustion of fossil fuel is sulphur dioxide. SO2 is a toxic gas, which is directly harmful to human health. It is heavier than air and has a suffocating odour at an atmospheric concentration of around 500 parts per billion (ppb), at which level it can be fatal.  At lower levels, chest pains, breathing problems, eye irritation and a lowered resistance to heart and lung diseases can be experienced. At 20 ppb or lower there should be no ill effects to a healthy person. The normal atmospheric background concentration of SO2 is generally less than 10 ppb. [3]

A secondary effect is the formation of sulphates (and nitrates), in the form of aerosols or very fine airborne particles, which can comprise a significant proportion of the particulate matter and have been linked to increased asthma attacks, heart and lung disease and respiratory problems in susceptible population groups. [3]

A third effect can occur further away from the emission source where the sulphur oxides will have converted to acids by aqueous phase reactions in the atmosphere. These acidic aerosols are eventually precipitated as acid rain, snow, sleet or fog but only when they encounter the right meteorological conditions. In the absence of man made pollution rain water would be slightly acidic, around pH 5, due to the presence of carbonic acid from the interaction of water vapour and naturally occurring levels of CO2. Acid rain on the other hand has been measured with pH levels below 3 corresponding to vinegar. [3]

The effects of acid deposition depend on the fragility of the materials, plants, soils and waters involved. In those instances where there is not the natural alkalinity to neutralise this acidity or alternatively the capability to withstand such attack, acid deposition has been linked with the acidification of ground and surface water, deforestation, reduction – even elimination – of aquatic life and building decay. The exposed surface of limestone (CaCO3) used for the fabric of many historical buildings turns to gypsum (CaSO4), which has a lower density and is more water soluble, hence suffers from spalling). The impacts on the natural environment can cause further problems for example, acidic water leeches out heavy metals. Soil no longer bound by tree roots, may be washed away, leaving a denuded landscape and potentially adding to the flood risk downstream where this soil is subsequently deposited as silt. [3]

In their 2009 joint proposal to the IMO, the USA and Canada stated that by designating the eastern and western seaboards of North America an Emissions Control Area, “as many as 8,300 lives will be saved and over three million people will experience relief from acute respiratory symptoms each year”. It was also stated “an ECA will result in a 19 per cent reduction in excess [sulphur and nitrogen] deposition in south-western British Columbia and it will eliminate excess deposition over about 13,500 km2 across Canada”. [5], [6], [7]

Relevant chemistry